1. Field of the Invention
The invention relates to a deceleration control apparatus and method for a vehicle. More particularly, the invention relates to a deceleration control apparatus and method for a vehicle, which controls the deceleration of the vehicle by operation of a brake system which applies braking force to the vehicle and a shift operation that shifts an automatic transmission into a relatively lower speed or speed ratio.
2. Description of the Related Art
Deceleration control is known which performs both a downshift in an automatic transmission and operation of a brake system so that the distance between a host vehicle and a preceding vehicle does not become equal to, or less than, a predetermined value. JP(A) 2001-30792 discloses technology which, when a target deceleration can not be achieved by fully closing the throttle valve and the downshift alone, achieves the target deceleration by fully closing a throttle valve and operating an automatic brake without performing a downshift, which improves riding comfort by avoiding shift shock generated by the downshift. Further, when the target deceleration is higher than a predetermined deceleration, it is regarded as an emergency so the deceleration control simultaneously fully closes the throttle valve, executes a downshift, and operates the automatic brake. (When the deceleration is referred to in this specification, it is understood to be high when the absolute value of the deceleration is large and low when the absolute value of the deceleration is small.)
Patent No. 3123384 discloses a deceleration control by a downshift of a transmission (i.e., downshift deceleration control) which is executed when the distance between vehicles is small. According to this technology, when the distance between vehicles (hereinafter, also referred to as “vehicle-to-vehicle distance”) becomes even smaller, a deceleration control by braking the wheels (i.e., braking deceleration control) is executed together with the downshift deceleration control. When this braking deceleration control is started within a predetermined period of time after the downshift deceleration control has started, however, the downshift deceleration control is cancelled by canceling means. As a result, deceleration control is performed by only braking the wheels so no sense of discomfort is imparted to the driver and good running is able to be achieved.
In the deceleration control, there are advantages and disadvantages to both the shift control that shifts the transmission into a lower speed and the brake control that operates the brake system. Shift control is advantageous in that the engine braking force increases steadily. A disadvantage of the shift control, on the other hand, is that response and controllability are poor. In comparison, brake control is advantageous in that it offers good response and controllability. The disadvantage of brake control, however, is that, from the viewpoint of durability and reliability, the brakes can not be continually applied for an extended period of time.
The technology disclosed in JP(A) 2001-30792 only executes a downshift and brake control simultaneously in cases of emergency because doing so adversely effects drivability. The technology disclosed in Patent No. 3123384 cancels the deceleration control by a downshift when the brake control starts.
Neither of the technologies described above simultaneously and actively execute a downshift and brake control, and thus do not incorporate all of the advantages (the good response and controllability of brake control and the steady increase in engine braking force of a downshift) of executing a downshift and brake control simultaneously. In order to take full advantage of the advantages of both shift control and brake control, it is desirable to perform deceleration control that executes shift control and brake control simultaneously, but yet does not impart an unpleasant sensation to the driver.